Various glass compositions suitable for forming glass fibers are known in the art, as discussed by K. L. Loewenstein in his book entitled "The Manufacturing Technology of Continuous Glass Fibers" (Elsevier Science Publishing Company, 1973), especially in Chapter IV on glass manufacturing. One desirable characteristic of such compositions is a resistivity to devitrify or crystallize during the fiber forming process. A devitrification resistant glass is one generally having a liquidus temperature significantly lower than the glass forming temperature, i.e., the temperature at which the glass has a viscosity (in poise) of log 2.5. This difference in temperature between the glass forming temperature and the liquidus temperature is called the "delta T." E-type glasses are an example of such a devitrification resistant glass.
U.S. Pat. No. 2,571,074 discloses a specific devitrification resistant E glass having the composition of 52-56% SiO.sub.2, 12-16% Al.sub.2 O.sub.3, 19-25% CaO and 8-13% B.sub.2 O.sub.3. However, certain melting inefficiencies are realized when these formulations are employed. For example, the melting rates are slow and the resulting throughput values are low when these batch compositions are employed. Also, because an alkali component is lacking, these formulations are not amenable to electric melting, which generally enhances melting efficiencies. Additionally, batch costs of these glass compositions are high due to the high content of B.sub.2 O.sub.3, the most expensive batch component.
C-type glasses, including the composition disclosed in U.S. Pat. No. 2,308,857, contain alkali and generally demonstrate good melting characteristics, however, they are not as durable in water as are other glasses, including E-type glases, and they also contain a substantial amount of B.sub.2 O.sub.3 making the batch cost expensive.
The present invention provides an alumino-borosilicate glass composition containing alkali which is water resistant and devitrification resistant and which permits certain melting efficiencies to be realized when employing the formulations of the instant invention.